Design, fabrication and characterizations of AlGaN/GaN heterostructure sensors
Sun, Jianwen
DOI
10.4233/uuid:e8068b85-f8f6-4ebd-951f-e9216eb2cc2c
Publication date
2020
Document Version
Final published version
Citation (APA)
Sun, J. (2020). Design, fabrication and characterizations of AlGaN/GaN heterostructure sensors.
https://doi.org/10.4233/uuid:e8068b85-f8f6-4ebd-951f-e9216eb2cc2c
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D
ESIGN
, F
ABRICATION AND
C
HARACTERIZATIONS
D
ESIGN
, F
ABRICATION AND
C
HARACTERIZATIONS
OF
A
L
G
A
N/G
A
N H
ETEROSTRUCTURE
S
ENSORS
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. dr. ir. T.H.J.J. van der Hagen, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op donderdag 25 juni 2020 om 12:30 uur
door
Jianwen S
UN
Master of Engineering in Integrated Circuit Engineering, Tsinghua University, Beijing, China,
Samenstelling promotiecommissie:
Rector magnificus, voorzitter
Prof. dr. ir. P. M. Sarro Technische Universiteit Delft, promotor Prof. dr. ir. G. Q. Zhang Technische Universiteit Delft, promotor Onafhankelijke leden
Prof. dr. ir. R. H. J. Fastenau Technische Universiteit Delft Prof. dr. P. J. French Technische Universiteit Delft Prof. dr. J. A. Ferreira Technische Universiteit Twente Prof. dr. X. J. Fan Lamar University
Prof. dr. Y. F. Qiu Xiamen University
Keywords: AlGaN/GaN, HEMT, MEMS, Micro-heater, Pressure sensor, UV sensor, Gas sensor
Printed by: GILDEPRINT printing Front & Back: Jianwen Sun
Copyright © 2020 by Jianwen Sun ISBN 978-94-6402-350-3
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any other means, electronic or mechanical, in-cluding photocopying, recording or by any other information storage and retrieval sys-tem, without the prior permission of the author.
An electronic version of this dissertation is available at
To all those who have helped me. Jianwen Sun
C
ONTENTS
Summary ix Samenvatting xi 1 Introduction 1 1.1 Background. . . 2 1.2 Wideband Semiconductor . . . 31.3 AlGaN/GaN Heterostructure Sensors . . . 4
1.3.1 Gas Sensing . . . 4
1.3.2 Ultraviolet (UV) Detection. . . 5
1.3.3 Pressure Sensing. . . 6
1.4 Research Objective . . . 7
1.5 Thesis Structure. . . 8
References. . . 9
2 Basic Theory of AlGaN/GaN Heterostrucutre Sensors 13 2.1 Introduction . . . 14
2.2 GaN Properties . . . 14
2.2.1 Polarization Effects in GaN Semiconductors. . . 15
2.2.2 2DEG Forming. . . 18
2.3 AlGaN/GaN Heterostructure Sensors . . . 20
2.3.1 Surface Chemistry and Gas Sensing Mechanisms . . . 20
2.3.2 UV detection. . . 23
2.3.3 Pressure sensing. . . 25
2.4 Chapter Summary . . . 27
References. . . 27
3 Design and Fabrication of AlGaN/GaN Heterostructure Sensors 31 3.1 Introduction . . . 32
3.2 Design and Fabricaiton of the basic HEMT MEMS Sensors . . . 32
3.2.1 The Sensor Geometry Design . . . 32
3.2.2 Mask Design and Process Flow. . . 33
3.2.3 Substrate material Selection . . . 34
3.2.4 Device Ioslation . . . 36
3.2.5 Metalization . . . 37
3.2.6 Deep reaction ion etching (DRIE) . . . 40
3.3 Sensor Packaging . . . 40
3.3.1 Choice of Sensor Packaging . . . 43
3.3.2 Comparison of Two Packages . . . 43 vii
3.4 Electrical Characterization of the HEMT Sensor Platform. . . 45
3.4.1 Ohmic Contact Measurement . . . 45
3.4.2 Micro-heater Calibration and Self-heating Influence. . . 46
3.4.3 I-V Characteristics of AlGaN/GaN Sensor . . . 48
3.5 Chapter Summary . . . 52
References. . . 52
4 AlGaN/GaN Heterostructure for Gas Sensing 55 4.1 Introduction . . . 56
4.2 Nano WO3/AlGaN/GaN Gas Sensor. . . 57
4.2.1 Device fabrication . . . 57
4.2.2 Gas Testing Setup . . . 59
4.2.3 Improving NO2Limit of Detection. . . 60
4.2.4 Response to Acetone. . . 65
4.3 Enhanced Sensitivity Using a Two-step Gate Recess Technique. . . 69
4.3.1 Two-step Gate Recess Technique. . . 69
4.3.2 Pt-Gate recessed HEMT Characterization . . . 70
4.4 Chapter Summary . . . 71
References. . . 72
5 AlGaN/GaN Heterostructures Deep Ultraviolet Detectors 87 5.1 Introduction . . . 88
5.2 Deep UV Illumination of AlGaN/GaN Heterostructure Photodetectors . . . 90
5.2.1 Fabrication of Photodetectors . . . 90
5.2.2 Photodetector measurements . . . 91
5.2.3 Results and Discussion. . . 91
5.3 Suppression of Persistent Photoconductivity (PPC) Effect of AlGaN/GaN Photodetectors . . . 95
5.3.1 Persistent Photoconductivity (PPC) Effect . . . 95
5.3.2 DC Heating Mode . . . 95
5.3.3 Pulsed Heating Mode . . . 97
5.3.4 Short-time Mono-pulse Heating Reset Mode. . . 100
5.4 Chapter Summary . . . 109
References. . . 110
6 Low Power AlGaN/GaN MEMS Pressure Sensor 117 6.1 Introduction . . . 118
6.2 GaN-based MEMS Pressure Sensor . . . 118
6.2.1 Device Fabrication. . . 118
6.2.2 Pressure Measurement Setup . . . 120
6.2.3 Static Measurement . . . 120
6.2.4 Dynamic Measurement . . . 123
6.2.5 Working principle of pressure sensing . . . 126
6.3 Chapter Summary . . . 128
CONTENTS ix
7 Conclusions and Research outlook 131
7.1 Conclusions. . . 131
7.2 Research Outlook. . . 133
Acknowledgements 139
Curriculum Vitæ 141
S
UMMARY
The microelectronics industry, next to the powerful, continuously scaling of integrated circuits, is currently evolving in the diversification of integrated functions, generally re-ferred to as more than Moore (MtM). MtM concerns all technologies enabling micro-systems to be elevated to a higher integration level, and with small package size, lower power consumption and lower cost. Microelectromechanical (MEMS) are crucial within this development. While Si has proven to be the primary contestant in the MEMS sensor market, there is a growing need for sensors operating at conditions beyond the limits of Si. Si-based micro-sensors cannot operate in harsh environments such as high tempera-ture, high radiation, high pressure, and chemically corrosive conditions. Wide bandgap semiconductors such as Gallium Nitride (GaN) are potential candidates to replace sili-con due to their specific characteristics and proven performance in the power or LED applications. The research objective of this thesis is to develop a MEMS sensor platform utilizing GaN-based materials. The design, fabrication, packaging, and measurement of pressure, deep UV photodetector, and gas sensors are presented and discussed.
Due to the strong piezoelectric and spontaneous polarization effects in GaN and Al-GaN, a high sheet density of two-dimensional electron gas (2DEG) is formed at the in-terface between AlGaN and GaN layers. These epitaxial layers were grown on <111> sil-icon substrates by MOCVD. The fabrication process started with mesa isolation by ICP etching. Then, a process module was specifically developed for low ohmic contact resis-tance. Afterward, e-beam evaporation and lift-off process were employed for deposition and patterning of metal layers for an integrated micro-heater and the device intercon-nect layers. Furthermore, a DRIE process was tuned to form the membrane structure, using silicon oxide as a hard mask. The completed sensors were packaged into CFQN packages due to the stable output current observed, compared to the COB package. The membrane temperature was affected by not only the micro-heater heating but also by the self-heating of the sensor. The device is not affected when exposed to high relative humidity ambient and the saturated current temperature coefficient is -0.63 mA/mm*K. The AlGaN/GaN heterojunctions exhibit great potential for high performance sen-sors development due to high carrier density two-dimensional electron gas (2DEG) at the interface introduced by the strong polarization effect, which is sensitive to the changes in surface potential. AlGaN/GaN HEMT sensors integrated with micro-heater on the suspended membrane were investigated for gas detection. The adoption of nano WO3as
a functional layer result in the capability to detect low concentration of 100 ppb NO2/N2
at 300 °C. When exposed to a 1 ppm NO2gas, a high sensing sensitivity of 1.1 % with a
response (recovery) time of 88 seconds (132 seconds) is obtained. The WO3/AlGaN/GaN
chip was packaged and further tested to determine detection limit and response time. Besides the improvement of detection limit, the WO3nanofilm also improved the
sensi-tivity and selecsensi-tivity to acetone gas. At 300 °C, a drain current change∆IDSof 0.31 mA,
as well as a high sensitivity of 25.7 % for 1000 ppm acetone were observed. For 1000 ppm xi
acetone concentration tRes(tRec) reduced from 147 (656) s at VH= 3.5 V (210 °C) to 48
(319) s at VH= 4 V (300 °C). Moreover, the response to 1000 ppm acetone gas was
signif-icantly larger than for ethanol, ammonia and CO gases at the same 1000 ppm concen-tration. The temperature of the sensor can be adjusted by the integrated micro-heater. Transient measurements of the sensor showed stable operation and good repeatability at different temperatures.
Also, based on the newly introduce, high precision two-step gate recess technique, a suspended gate recess Pt/AlGaN/GaN heterostructure NO2gas sensor integrated with
a micro-heater was microfabricated. This gate recess technique dramatically enhances the performance of AlGaN/GaN devices. The sensitivity and current change of AlGaN/GaN heterostructure to 1-200 ppm NO2/air are increased up 20 times and 12 times
com-pared to conventional gate devices respectively with the faster response time. The sus-pended membrane structure and integrated micro-hotplate also improve response time and sensitivity by adjusting the optimum working temperature with low power con-sumption. The sensitivity (response time) to 40 ppm NO2/air increases from 0.75 %
(1250 s) to 3.5% (75 s) when temperature increases from 60 °C to 300 °C.
The AlGaN/GaN heterostructure UV photodetector with functionalized WO3nanofilm
deposited by PVD technology shows high response to the deep UV wavelength and ex-hibits high responsivity of 1.67×104A/W at 240 nm, and a sharp cut-off wavelength is 275 nm. The long decay time of photodetector introduced by the PPC effect was opti-mized by three kinds of heating methods. The photodetector shows a rapid response and recovery (175 s) time under 240 nm illumination at the DC heating temperature of 150 °C. To further reduces the decay time of the AlGaN/GaN heterostructure photode-tectors, a reduction of 30-45 % in decay time is measured by 50 Hz pulsed heating mode compared to DC heating. More importantly, the PPC effect can be eliminated by a novel method: mono-pulse heating reset (MHR) by applying a pulse voltage of micro-heater after the removal of UV illumination. The recovery time was reduced from hours to sec-onds without reducing the high responsivity and stability of photodetector. This novel method solved the long-term problem of long decay time introduced by PPC of GaN-based photoconductive photodetectors.
The piezoresistive gauge factor of AlGaN/GaN heterostructures is approximately three times higher than the highest gauge factor reported for SiC. The chip was bonded to a second silicon wafer using silicone (BISON) to create a reference pressure inside. The suspended membrane AlGaN/GaN heterostructure sensor showed a rapid response in drain current change when exposed to different vacuum pressures, especially in low pressure range. And the higher temperature increased the sensitivity due to the larger deflection of the membrane at the higher temperature. The dynamic percent current change of the AlGaN/GaN heterostructure pressure sensor was 18.75 % under pressure of 10 Pa at 100 °C with a low operating power consumption of 1.8µW. The maximum sensitivity was obtained as 22.8 %/kPa with pressure ranging from 600 Pa to 10 Pa.
S
AMENVAT TING
De industrie van micro-elektronica ontwikkelt zich vandaag de dag – behalve op het gebied van de schaal van geïntegreerde circuits – ook in de richting van de veelzijdig-heid in geïntegreerde functies, meestal More than Moore (MtM) genoemd. MtM omvat alle technologieën die microsystemen naar een hoger niveau van integratie tillen, met kleinere pakketvolume (package size), lager verbruik en lagere kosten. Micro-elektro-mechanische systemen (MEMS) zijn bij deze ontwikkeling cruciaal. Hoewel er aange-toond is dat Si vooraanstaand is op de markt van MEMS-sensoren, is er een toenemende vraag naar sensoren die werkzaam zijn onder toestanden die buiten de capabiliteit van Si vallen. Si-gebaseerde microsensoren kunnen niet functioneren in barre omgevingen met bijvoorbeeld hoge temperatuur, veel straling, hoge druk en chemisch corrosieve middelen. Halfgeleiders met een brede bandkloof zoals galliumnitride (GaN) zijn po-tentiële kandidaten om silicium te vervangen vanwege hun specifieke karakteristieken en het aantoonbare functioneren in elektriciteitsnet- of LED-toepassingen. Deze disser-tatie heeft als doel om een MEMS-sensorplatform te ontwikkelen met het gebruik van GaN-gebaseerde materialen. Het ontwerp, de fabricage, verpakking (packaging), druk-meting, diep-UV fotodetector en gassensoren worden voorgelegd en bediscussieerd.
Vanwege de sterke piëzo-elektrische en spontane polarisatie-effecten in GaN en Al-GaN, wordt tweedimensionaal elektrongas (2DEG) met een hoge plaatdichtheid gevormd op het grensvlak tussen AlGaN- en GaN-lagen. Deze epitaxiale lagen zijn door MOCVD op <111> siliciumsubstraten gekweekt. Het fabricageproces begon met mesa-isolatie door ICP-etsen. Vervolgens is een procesmodule speciaal ontwikkeld voor lage ohmse contactweerstanden. Daarna werden e-beam verdampings- en liftoff-processen gebruikt voor het afzetten en vormgeven van metalen lagen voor een geïntegreerde microverwar-mer en de apparaatverbindingslagen. Verder werd een DRIE-proces afgestemd om de membraanstructuur te vormen, met SiO2als een hard masker. De voltooide sensoren
zijn verpakt in CFQN-pakketten vanwege de stabiele waargenomen uitgangsstroom in vergelijking met het COB-pakket. De membraantemperatuur wordt niet alleen beïn-vloed door de opwarming van de microverwarming, maar ook door de zelfverwarming van de sensor. Het apparaat wordt niet beïnvloed bij blootstelling aan een hoge rela-tieve vochtigheidsomgeving en de verzadigde huidige temperatuurcoëfficiënt is -0,63 mA/mm*K.
De AlGaN/GaN-heterojuncties vertonen een groot potentieel voor de ontwikkeling van hoogwaardige sensoren als gevolg van tweedimensionaal elektrongas (2DEG) met hoge dragerdichtheid aan de interface die wordt geïntroduceerd door het sterke polarisatie-effect, dat gevoelig is voor de veranderingen in oppervlaktepotentiaal. AlGaN/GaN HEMT-sensoren geïntegreerd met microverwarming op een hangend membraan werden on-derzocht op gasdetectie. De toepassing van nano-WO3als functionele laag resulteert
in het vermogen om een lage concentratie van 100 ppb NO2/N2bij 300 °C te
detecte-ren. Bij blootstelling aan een NO2-gas van 1 ppm wordt een hoge detectiegevoeligheid
van 1,1 % bereikt met een respons (herstel) tijd van 88 seconden (132 seconden). De WO3/AlGaN/GaN-chip werd verpakt en verder getest om de detectielimiet en
reactie-tijd te bepalen. Naast de verbetering van de detectielimiet, verbeterde de WO3-nanofilm
ook de gevoeligheid en selectiviteit voor acetongas. Bij 300 °C werd een verandering in de drain-stroom∆IDSvan 0,31 mA waargenomen, evenals een hoge gevoeligheid van 25,7
% voor 1000 ppm aceton. Voor een acetonconcentratie van 1000 ppm verminderde tRes
(tRec) van 147 (656) s bij VH= 3.5 V (210oC) tot 48 (319) s bij VH= 4 V (300 °C). Bovendien
was de respons op 1000 ppm acetongas significant groter dan voor ethanol, ammoniak en CO-gassen bij dezelfde 1000 ppm-concentratie. De temperatuur van de sensor kan worden aangepast door de geïntegreerde microverwarmer. Voorbijgaande metingen van de sensor lieten een stabiele werking en goede herhaalbaarheid bij verschillende tempe-raturen zien.
Hierbij is, gebaseerd op de nieuw geïntroduceerde en zeer nauwkeurige tweestaps gate recess techniek, een hangende gate recess Pt/AlGaN/GaN heterostructuur NO2
-gassensor micro-gefabriceerd met een geïntegreerde microverwarmer. Deze gate re-cess techniek verbetert de prestaties van AlGaN/GaN-devices drastisch. De gevoelig-heid en de stroomsterkteverschil van de AlGaN/GaN-heterostructuur naar 1-200 ppm NO2/lucht worden tot 20 keer respectievelijk 12 keer verhoogd in vergelijking met
con-ventionele gate-apparaten, respectievelijk met de snellere responstijd. De hangende membraanstructuur en geïntegreerde microkookplaat verbeteren ook de reactietijd en gevoeligheid door de optimale werktemperatuur aan te passen met een laag stroomver-bruik. De gevoeligheid (reactietijd) tot 40 ppm NO2/lucht neemt toe van 0,75 % (1250 s)
tot 3,5 % (75 s) als de temperatuur stijgt van 60 °C tot 300 °C.
De AlGaN/GaN-heterostructuur UV-fotodetector met gefunctionaliseerde WO3-nanofilm
afgezet met PVD-technologie vertoont een sterke reactie op de verre UV-golflengte en vertoont een hoge responsiviteit van 1.67×104A/W bij 240 nm, en een scherpe cut-off golflengte van 275 nm. De lange vervaltijd van de fotodetector geïntroduceerd door het PPC-effect werd geoptimaliseerd door drie soorten verwarmingsmethoden. De foto-detector toont een snelle respons- en hersteltijd (175 s) onder een verlichting van 240 nm bij een gelijkstroom-verwarmingstemperatuur van 150 °C. Om de vervaltijd van de AlGaN/GaN-heterostructuur fotodetectoren verder te verminderen, wordt een vermin-dering van 30-45 % in vervaltijd gemeten met een gepulseerde verwarmingsmodus van 50 Hz, vergelijkbaar met de gelijkstroomverwarming. Een belangrijker punt is dat het persistente effect van fotoconductiviteit (PPC) kan worden geëlimineerd door een nieuwe methode: monopulsverwarmingsreset (MHR) door een pulsspanning van microverwar-ming toe te passen na het verwijderen van UV-verlichting. De hersteltijd werd terugge-bracht van uren naar seconden zonder de hoge reactiviteit en stabiliteit van de fotode-tector te verminderen. Deze nieuwe methode loste het langetermijnprobleem op van lange vervaltijd, geïntroduceerd door PPC van op GaN gebaseerde fotoconductieve fo-todetectoren.
De piëzoresistieve meetfactor van AlGaN/GaN-heterostructuren is ongeveer drie-maal hoger dan de hoogste meetfactor die is gerapporteerd voor SiC. De chip werd met silicium (BISON) aan een tweede siliciumwafer gehecht om een referentiedruk binnenin te creëren. De AlGaN/GaN heterostructuursensor met een hangend membraan toonde een snelle respons bij verandering van de drain-stroom bij blootstelling aan
ver-SAMENVATTING xv
schillende vacuümdrukken, met name in het lage drukbereik. De hogere temperatuur verhoogde de gevoeligheid door de grotere afbuiging van het membraan. De dynami-sche procentuele stroomverandering van de AlGaN/GaN-heterostructuur-druksensor was 18,75 % onder druk van 10 Pa bij 100 °C met een laag stroomverbruik van 1,8µW.
1
I
NTRODUCTION
1
1.1.
B
ACKGROUND
F
ORdecades the microelectronics industry has been fueled by Moore’s law and the outcome is today’s powerful integrated circuits (IC) with more functionalities and smaller sizes than ever before. Next to miniaturization also diversification is becoming more and more relevant to industry and academia. These are often referred to as More Moore and More than Moore (MtM),respectively[1,2]. MtM refers to all technologies enabling non digital functions (Analog/RF, HV power, microelectromechanical (MEMS) sensors and actuators, etc.) that dot not simply scale with Moore’s law, but provide addi-tional value in different ways, towards system on chip (SoC) and system in package (SIP). Such a combination of digital function with complementary non-digital content is de-picted in Figure1.1, illustrating the tendency of the micro-systems to higher integration, smaller package size, lower power consumption, and lower cost.Figure 1.1: The International Technology Roadmap for Semiconductors: miniaturization and functional diver-sification [3].
Micro-electro-mechanical systems (MEMS) refers to micro-scale devices offering at-tractive characteristics such as reduced size and weight, low power consumption and high speed and precision. MEMS sensors are employed in/targeting a wide variety of applications and can be divided into several types in accordance with the measured
1.2.WIDEBANDSEMICONDUCTOR
1
3
Table 1.1: Physical characeristics of Si and main wide bandgap semiconductors.
Property Si GaAs 4H-SiC GaN Diamond
Eg(eV) 1.12 1.43 3.26 3.40 5.47 Ec(kV/cm) 300 400 2200 3300 10000 µn(cm2V−1s−1) 1500 8500 1250 2000 (2DEG) 2000 λ (W/cm.K) 1.5 0.46 4.9 1.3 20 vsat(107cm/s) 1 1 2 2.2 2
Eg: Bandgap energy; Ec:Electric breakdown field;µn:Electron mobility;λ: Thermal
conductivity; vsat:Saturated electron drift velocity.
quantity, such as acceleration, pressure, displacement, flow, electromagnetic field, im-age, temperature, gas composition, and ionic concentration. The MEMS sensor, as a key device for information acquisition, plays a significant role in promoting the miniaturiza-tion of various sensor systems, which have been widely used in aerospace, biomedicine, and consumer electronics. At present, with the introduction of new materials such as nanomaterials, biological materials, and intelligent materials, in addition to the contin-uous development of nanomanufacturing technology, MEMS sensors are rapidly devel-oping into high precision, high-reliability, and multi-functional integrated system. In ad-dition, the emergence of the IOT has greatly increased the demands for wireless MEMS sensors, and reducing the power supply of these devices has become a key point.
1.2.
W
IDEBAND
S
EMICONDUCTOR
With the many years of development in silicon fabrication and processing techniques, Si-based MEMS sensors have the advantage of producing uniform, accurate micro/nano size structures, suitable for mass production. While Si has proven to be the primary ma-terial in the MEMS sensor market, there is a growing need for sensors operating at con-ditions beyond the limits of Si. Si-based micro-sensors can’t be operated in harsh en-vironments such as in high temperature, high radiation, high pressure, and chemically corrosive conditions. Developing sensors capable to operate in harsh environments is especially urgent. Appropriate materials for such environments are the key factor in whether the sensor will operate as designed over its required lifetime. During the design of devices for harsh environment, the properties of the materials used that are impor-tant to consider are the coefficient of thermal expansion, thermal conductivity, elastic modulus, and resistance to creep and fatigue.
A number of materials have been investigated as potential candidates to replace sili-con in electronic and MEMS devices in high-temperature or harsh environment applica-tions. A comparison among the characteristics of silicon and main wide bandgap semi-conductor is summarized in table 1-1. Wide bandgap semisemi-conductors (Eg> 2.3 eV) such
as Gallium Nitride (GaN), diamond, and Silicon Carbide (SiC) are potential candidates to attain superior and robust performances. The larger bandgap implies an ability to handle higher electric fields, thus enabling devices with higher operating temperatures (600 °C) at which conventional Si based sensors fail. An operation temperature limit up
1
to 175 °C for silicon sensors and 500 °C for silicon-on-insulator (SOI) sensors has beenreported [4]. The ability of GaN-based materials to function at high temperature, highpower and high radiation environment will enable large performance enhancements in a wide variety of applications, such as spacecraft [5][6], satellite [7][8], automobile[9], nuclear power and radar [10].
1.3.
A
L
G
A
N/G
A
N H
ETEROSTRUCTURE
S
ENSORS
In recent years, GaN has attracted great attention for both optoelectronics and electron-ics applications due to its superior material properties such as direct band gap, higher electric breakdown field strength, both higher electron mobility and saturation veloc-ity, high melting point and the ability to form a heterostructure. Especially, it is already commercialized in power switch devices, such as Schottky diodes and high electron mo-bility transistor (HEMT)[5]. The major advantage of using GaN-based materials, such as AlGaN and InGaN, is the formation of the heterostructure which results in the cre-ation of a two-dimensional electron gas (2DEG)[11], where energy states for electrons are quantized and electrons can only move laterally. Very high electron mobility can be achieved in AlGaN/GaN heterostructures, since carriers are screened from their respec-tive donors, mitigating ionized impurity scattering. The ability to form a heterostruc-ture and the fact that GaN epitaxial layers can be grown on different, affordable such as Si, and sapphire are additional advantages. The high piezoelectric gauge factor of GaN makes them ideal for pressure and stress detection[12]. The wide energy bandgap of GaN and AlGaN make these materials suitable for UV detection[15,16]. And the 2DEG is sensitive to changes of surface states, which indicates the AlGaN/GaN heterojunctions great potential for chemical sensors[13], such as gas sensors and PH sensors as well as for biological sensors. Among all possible applications, three very promising ones are gas sensing, UV detection and pressure sensing.
1.3.1.
G
ASS
ENSINGGas sensors are increasingly used in the growing markets of automotive, aerospace, health care, environment protection, consumer products. Within these domains, gas sensors play an important role in providing comfort and safety or in enabling process control or smart maintenance functionalities. As shown in Figure1.2, Yole Development’s Gas Sensor report estimates that the gas sensor market is currently growing in most of these applications areas. Moreover, in the essential pursue of improving sensitivity and selec-tivity of gas detection, it should be noticed that these various applications require very different levels of sensor performance.
In spite of the progress achieved with silicon-based gas sensors in most of these ap-plications, they are not suitable for operation in corrosive environments and high tem-peratures (>250 °C) due to the narrow bandgap of silicon. Operation in harsh environ-ments is essential for industrial manufacturing, powertrain, and automotive industries. Therefore, wide bandgap semiconductor-based gas sensors are required. SiC-based gas sensors have already shown their high temperature properties. However, the cost of the SiC substrate is still very high. GaN is a highly promising material for harsh environ-ments. Especially the AlGaN/GaN heterostructures form a highly conductive electron
1.3.ALGAN/GAN HETEROSTRUCTURESENSORS
1
5
Figure 1.2: 2018-2023 gas sensor market in value ($B). (source: Yole development November 2018)
channel, and this channel, as mentioned earlier, is very sensitive to the surface state. Consequently, the potential of these structures for a high sensitivity gas sensor with high stability for harsh environments is significant.
1.3.2.
U
LTRAVIOLET(UV ) D
ETECTIONAn ideal UV detector should work in a radiation environment and not be susceptible to long-wave electromagnetic interference. It does not detect signals to targets by actively radiating electromagnetic waves outwards but it recognizes signals by passively receiv-ing ultraviolet radiation. UV photodetectors have a wide range of applications in military and civilian fields as shown in Figure1.3, such as missile warning, fire warning, marine oil pollution detection, biomedical detection, and environment UV detection.
The representative UV detector in the early stage is the Photomultiplier tube (PMT). PMT UV photodetectors with unique features such as high stability, high sensitivity, high speed, the high signal-to-noise ratio are generally bulky, fragile and require high bias voltages.[14] Moreover, the growing needs and expectations are exceeding the per-formances of traditional UV photodetectors, and UV photodetectors with some special function and multiple functions have become more and more essential for practical ap-plications. More compact and smart UV photodetectors are urgently needed to guaran-tee high performance in the future.
In order to meet the requirement of compactness, silicon-based UV photodetectors were considered due to their small size, high integration level, suitability for large area arrays, and low cost. However, the optical spectral response of the silicon photodetector is in the infrared region due to the narrow bandgap (1.12 eV). Also, the responsivity of silicon to UV range is low and its stability is not high. In real application, the silicon UV photodetector needs an expensive infrared filter to remove the interference of visible and infrared light. In addition, the low UV absorption of silicon makes the quantum efficiency of silicon photodetector very low and exhibits poor radiation resistance.
1
Figure 1.3: Typical applications of UV photodetector.
have high chemical bonding structures and intrinsic visible-blindness. However, the wide bandgap semiconductor-based photodetectors have not been sufficiently exploited so far due to insufficient materials and technology maturity. With the progress recently made in the development of wide bandgap semiconductors materials and technology, wide bandgap semiconductor-based UV photodetector inject new vitality to the research and development of high-performance UV photodetectors. In recent years, GaN based UV light detectors have been reported [15,16]. GaN is particularly suitable for UV light detection due to its direct wide band gap and robust nature. GaN PIN diodes [17,18], Schottky diodes [19] and metal semiconductor metal (MSM) [20–22] based photo de-tectors have been demonstrated by various researches. From the aspect of technology, the ideal photodetector would exhibit lower dark current to minimize the interference noise and higher responsivity to maximize the photo signal. Currently, the avalanche-type detector [23–25] can obtain high responsivity but at the expense of increased noise, highly rigorous requirements of structure and processing techniques. Another common approach to improve responsivity is a photoconductive type, which is easy to fabricate at lower cost and has a good commercial prospect. However, persistent photoconductivity (PPC) effect associated with a 2DEG in an AlGaN/GaN HEMT devices has been observed. That is a big challenge that needs to be addressed to develop a UV photodetector with high responsivity and fast response at the same time.
1.3.3.
P
RESSURES
ENSINGThere are lots of applications in the automotive, medical, consumer, aerospace, and in-dustrial fields for robust miniaturized pressure sensors as illustrated in Figure1.4. Auto-motive is the largest market by far and with significant growth. The consumer market is much smaller due to low pricing but grows considerably . Avionics & high end is a niche market but with high growth. Today, MEMS pressure sensor technologies are quite
ma-1.4.RESEARCHOBJECTIVE
1
7
ture for most of these applications and are basically separated into piezoresistive and ca-pacitive types. The piezoelectric is leading in terms of market share. And most of MEMS pressure sensors are based on silicon technology. In high end applications, the pressure sensor need to operate at elevated temperature (>150 °C). While many papers have been published about this, there are no commercial semiconductor-based sensors for tem-perature ranges above 250 °C. Numerous solid-state pressure sensors are based on the mechanism of the piezoelectric effect [26–31]. Among them, silicon-on-insulator (SOI) [32] and silicon carbide (SiC) [33–35] based pressure platforms are the most promising technology. While GaN is a less mature technology than SiC, an advantage of GaN-based devices is their potential for monolithic integration. The piezoresistive gauge factor of AlGaN/GaN heterostructures is approximately three times higher than the highest gauge factor reported for SiC. It means that the AlGaN/GaN heterostructure could be used for pressure or stress sensors. In addition, the demonstration of sensing using AlaN/GaN platform for vacuum application has been hardly reported.
Figure 1.4: MEMS pressure sensor applications. (source: Yole Development 2018)
1.4.
R
ESEARCH
O
BJECTIVE
Although AlGaN/GaN HEMTs have been prevailing in microwave and power electron-ics, their potential in sensing applications is not fully developed yet. In order to expand the sensing applications of GaN devices and meet the requirements of next generation sensors (low cost, high volume, highly miniaturized, high reliability and low power con-sumption), the objective of this thesis is to develop a MEMS sensor platform utilizing GaN-based materials. As first step, gas sensing, deep UV photodetector and vacuum pressure sensor, are targeted.
men-1
tioned goals:1. Select the optimal substrate and epitaxial structure for MEMS AlGaN/GaN sensor fabrication.
2. Design, fabricate and package the MEMS AlGaN/GaN sensor platform.
3. Gas detection: Test gas sensor response to NO2and acetone and optimize the
dy-namic performance by functional materials and chip-level heating unit. Investi-gate the gas response effect of Investi-gate recess on AlGaN/GaN heterostructure sensor performance.
4. UV detection: Measure the UV response and optimize the dynamic performance of the WO3/AlGaN/GaN heterostructure photodetectors by an integrated
micro-heater.
5. Pressure sensor: test vacuum static and dynamic performance and investigate the temperature effects on MEMS AlGaN/GaN heterostructure sensor.
1.5.
T
HESIS
S
TRUCTURE
The rest of this thesis is structured as indicated by the block diagram shown in Figure 1.5.
Chapter 2 focuses on the theory of AlGaN/GaN heterostructure sensors, including the polarization effect in GaN semiconductors and the 2DEG formation. Afterward, the piezoelectric, optical and chemical sensing mechanisms of AlGaN/GaN heterostructure sensors are presented.
Chapter 3 presents the base design, simulation and fabrication process of the MEMS AlGaN/GaN heterostructure device. Then, the sensor package is discussed and the wafer-level testing including ohmic contact, temperature and humidity effect on the device are measured and discussed.
Chapter 4 shows the application of the AlGaN/GaN heterostructure sensor to gas sensing. A WO3nanolayer deposited by physical vapor deposition (PVD) on the gate is
studied as a way to improve selectivity. A two-step method to precisely etch the AlGaN layer is developed and the gate recess AlGaN/GaN sensor transient response to different gases is compared.
Chapter 5 describes the application of the AlGaN/GaN heterostructure device as a deep ultraviolet photodetector. The mechanism of the GaN-based photoconductive de-tector and its persistent photoconductivity (PPC) effect are studied and discussed. Three methods, DC heating, pulse heating and mono-pulse heating reset (MHR) to suppress the PPC effect are investigated.
Chapter 6 reports on the investigation of the application the AlGaN/GaN MEMS het-erostructure sensor for pressure sensing, especially the vacuum range. The deformation of the GaN membrane is discussed. Static and dynamic sensor response characteristics are studied under different temperatures, and the temperature effect on the sensitivity of the sensor is assessed.
Chapter 7 summarizes the result of this research and draws some conclusions on the potential of AlGaN/GaN heterostructures for sensing. The achievements of this thesis
REFERENCES
1
9
Figure 1.5: The block diagram of this thesis structure.
open up an opportunity for future potential applications as well as further technology development. A brief outlook for future research is also given.
R
EFERENCES
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[2] G.Q. Zhang, Alfred van Roosmalen. More than Moore - Creating High Value Mi-cro/Nanoelectronics Systems. ISBN: 978-0-387-75592-2, Springer. 2009
[3] W. Arden, M. Brillouët, P. Cogez, M. Graef, B. Huizing, and R. Mahnkopf, "More-than-Moore white paper," Version, vol. 2, p. 14, 2010.
[4] M. A. Fraga, R. S. Pessoa, H. S. Maciel, and M. Massi, "Recent developments on silicon carbide thin films for piezoresistive sensors applications," Silicon Carbide. Rijeka: Intech-Open Acess Publisher, vol. 1, pp. 369-388, 2011.
[5] Y. Kobayashi et al., "GaN HEMT based rectifier for spacecraft health monitoring system using microwave wireless power transfer," in 2012 Asia Pacific Microwave Conference Proceedings, 2012, pp. 391-393: IEEE.
[6] S. Yoshida, N. Hasegawa, and S. Kawasaki, "Experimental demonstration of mi-crowave power transmission and wireless communication within a prototype reusable spacecraft," IEEE Microwave and Wireless Components Letters, vol. 25, no. 8, pp. 556-558, 2015.
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[9] R. Ma, K. H. Teo, S. Shinjo, K. Yamanaka, and P. M. Asbeck, "A GaN PA for 4G LTE-Advanced and 5G: Meeting the telecommunication needs of various vertical sectors including automobiles, robotics, health care, factory automation, agriculture, edu-cation, and more," IEEE Microwave Magazine, vol. 18, no. 7, pp. 77-85, 2017. [10] P. Blount, S. Huettner, and B. Cannon, "A High Efficiency, Ka-Band Pulsed Gallium
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[12] M. S. Shur, A. D. Bykhovski, and R. Gaska, "Pyroelectric and piezoelectric properties of GaN-based materials," (in English), Mrs Internet Journal of Nitride Semiconduc-tor Research, vol. 4, 1999.
[13] B. S. Kang, K. Suku, F. Ren, B. P. Gila, C. R. Abernathy, and S. J. Pearton, "AlGaN/GaN-based diodes and gateless HEMTs for gas and chemical sensing," IEEE Sensors Jour-nal, vol. 5, no. 4, pp. 677-680, 2005.
[14] A. Bouvier et al., "Photosensor characterization for the Cherenkov Telescope Array: silicon photomultiplier versus multi-anode photomultiplier tube," in Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XV, 2013, vol. 8852, p. 88520K: Interna-tional Society for Optics and Photonics.
[15] E. Munoz, E. Monroy, J. L. Pau, F. Calle, F. Omnes, and P. Gibart, "III nitrides and UV detection," (in English), Journal of Physics-Condensed Matter, vol. 13, no. 32, pp. 7115-7137, Aug 13 2001.
[16] P. E. Malinowski et al., "Backside-Illuminated GaN-on-Si Schottky Photodiodes for UV Radiation Detection," (in English), Ieee Electron Device Letters, vol. 30, no. 12, pp. 1308-1310, Dec 2009.
[17] B. Butun, T. Tut, E. Ulker, T. Yelboga, and E. Ozbay, "High-performance visible-blind GaN-based p-i-n photodetectors," (in English), Applied Physics Letters, vol. 92, no. 3, p. 033507, Jan 21 2008.
[18] T. Tut, T. Yelboga, E. Ulker, and E. Ozbay, "Solar-blind AlGaN-based p-i-n photode-tectors with high breakdown voltage and detectivity," (in English), Applied Physics Letters, vol. 92, no. 10, p. 103502, Mar 10 2008.
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[19] K. H. Lee, P. C. Chang, S. J. Chang, Y. C. Wang, C. L. Yu, and S. L. Wu, "AlGaN/GaN Schottky Barrier UV Photodetectors With a GaN Sandwich Layer," (in English), Ieee Sensors Journal, vol. 9, no. 7, pp. 814-819, Jul 2009.
[20] C. K. Wang et al., "GaN MSM UV photodetectors with titanium tungsten transparent electrodes," (in English), Ieee Transactions on Electron Devices, vol. 53, no. 1, pp. 38-42, Jan 2006.
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[22] C. K. Wang et al., "GaN MSM UV Photodetector With Sputtered AlN Nucleation Layer," (in English), Ieee Sensors Journal, vol. 15, no. 9, pp. 4743-4748, Sep 2015. [23] J. Zheng et al., "A PMT-like high gain avalanche photodiode based on GaN/AlN
pe-riodically stacked structure," Applied Physics Letters, vol. 109, no. 24, p. 241105, 2016.
[24] C. Bayram, J. Pau, R. McClintock, and M. Razeghi, "Performance enhancement of GaN ultraviolet avalanche photodiodes with p-type -doping," Applied Physics Let-ters, vol. 92, no. 24, p. 241103, 2008.
[25] Q. Cai et al., "AlGaN ultraviolet Avalanche photodiodes based on a triple-mesa structure," Applied Physics Letters, vol. 113, no. 12, p. 123503, 2018.
[26] Y. Watanabe, S. Uemura, and S. Hoshino, "Printed pressure sensor array sheets fab-ricated using poly (amino acid)-based piezoelectric elements," Japanese Journal of Applied Physics, vol. 53, no. 5S3, p. 05HB15, 2014.
[27] J. Yoo et al., "Piezoelectric and dielectric properties of La2O3 added Bi (Na, K) TiO3–SrTiO3 ceramics for pressure sensor application," Sensors and Actuators A: Physical, vol. 126, no. 1, pp. 41-47, 2006.
[28] R. Bao et al., "Flexible and controllable piezo-phototronic pressure mapping sensor matrix by ZnO NW/p-polymer LED array," Advanced Functional Materials, vol. 25, no. 19, pp. 2884-2891, 2015.
[29] D. Mandal, S. Yoon, and K. J. Kim, "Origin of piezoelectricity in an electrospun poly (vinylidene fluoride-trifluoroethylene) nanofiber web-based nanogenerator and nano-pressure sensor," Macromolecular rapid communications, vol. 32, no. 11, pp. 831-837, 2011.
[30] M. Peng et al., "High-resolution dynamic pressure sensor array based on piezo-phototronic effect tuned photoluminescence imaging," ACS nano, vol. 9, no. 3, pp. 3143-3150, 2015.
[31] P. Khanna, B. Hornbostel, R. Grimme, W. Schäfer, and J. Dorner, "Miniature pressure sensor and micromachined actuator structure based on low-temperature-cofired ceramics and piezoelectric material," Materials chemistry and physics, vol. 87, no. 1, pp. 173-178, 2004.
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[32] S. Guo, H. Eriksen, K. Childress, A. Fink, and M. Hoffman, "High temperature smart-cut SOI pressure sensor," Sensors and Actuators A: Physical, vol. 154, no. 2, pp.255-260, 2009.
[33] R. S. Okojie, D. Lukco, V. Nguyen, and E. Savrun, "4H-SiC piezoresistive pressure sensors at 800 C with observed sensitivity recovery," IEEE Electron Device Letters, vol. 36, no. 2, pp. 174-176, 2014.
[34] C. A. Zorman and R. J. Parro, "Micro-and nanomechanical structures for silicon car-bide MEMS and NEMS," physica status solidi (b), vol. 245, no. 7, pp. 1404-1424, 2008.
[35] L. Chen and M. Mehregany, "A silicon carbide capacitive pressure sensor for in-cylinder pressure measurement," Sensors and Actuators A: Physical, vol. 145, pp. 2-8, 2008.
2
B
ASIC
T
HEORY OF
A
L
G
A
N/G
A
N
H
ETEROSTRUCUTRE
S
ENSORS
2
2.1.
I
NTRODUCTION
This chapter discusses the properties of GaN materials and AlGaN/GaN heterostruc-tures, the 2DEG formation, and the operation principles of basic AlGaN/GaN devices. Then the mechanism and structures of the developed AlGaN/GaN heterostructure based sensors, gas sensors, UV detectors, and pressure sensors, are presented.
2.2.
G
A
N P
ROPERTIES
Gallium nitride (GaN) was first synthesized using hydride vapor phase epitaxy (HVPE) in 1969 [1], and was identified as a direct bandgap semiconductor (3.4 eV). But GaN started to attract attention only when a suitable metal organic chemical vapor deposition (MOCVD) equipment was developed in 1991 by Nakamura [2,3]. GaN semiconductors can grow with Zinc blende and Wurtzite crystal formation on a variety of substrates such Silicon (Si), silicon carbide (SiC) and sapphire. However, the ternary alloys of GaN (such as AlxGa1−xN and InxGa1−xN) possess a wurtzite crystal structure, which is
thermody-namically the most stable phase under ambient conditions. In this thesis, we focus on properties of wurtzite GaN. The wurtzite crystal structure is formed by hexagonal unit cells which consist of two intercepting hexagonal close-packed (hcp) sublattices. The base lattice constant and the height of the cell are a0and c0as shown in Figure2.1.
Figure 2.1: Hexagonal wurtzite Ga-face terminated GaN lattice structure[4].
The electromechanical properties of GaN and other semiconductors are summa-rized in Table 2-1. The piezoelectric coefficient of GaN is about three times larger than SiC and GaAs, which means that GaN is suitable for piezoelectric devices. From the me-chanical parameters of GaN, such as Young’s modulus, it can be seen that GaN is one of
2.2.GAN PROPERTIES
2
15
Table 2.1: Electromechanical properties of semiconductor materials.[5]
Materials Elastic Modulus c33(GPa) Acoustic Velocity (m/s) Piezoelectric Coefficient e33(C m−2) Young’s Mod-ulus (Gpa) Si 165 8415 N/A 130-187 SiC 605 13100 0.2 450 GaAs 118 2470 -0.16 85.5 AlN 390 11000 1.55 344.83 LiNbO3 60 3900 3.65 170 GaN 398 8044 0.65 210-405
Table 2.2: Spontaneous polarization coefficients and lattice constants of GaN and AlN. [6]
Parameters GaN AlN
Psp(C/m2) -0.029 -0.081
a(Å) 3.189 3.112
c(Å) 5.158 4.982
the most promising materials for electromechanical system.
2.2.1.
P
OLARIZATIONE
FFECTS ING
AN S
EMICONDUCTORSDue to the electronegativity difference of bonded atoms, the chemical bonds of com-pound semiconductors are both covalent and ionic together. The unique characteristic of group III-Nitride semiconductors are related to the presence of the large electronega-tivity of nitrogen. The GaN wurtzite structure is non-centrosymmetric along the c-axis or [0001] direction, which results in polarization along this axis, namely spontaneous po-larization. The spontaneous polarization (Psp) is present without any external
mechani-cal and electrimechani-cal perturbation. The magnitude of spontaneous polarization depends on the c0/a0ratio. The spontaneous polarization coefficients of GaN, AlN along with lattice
parameters are shown in Table 2-2.
The lattice constants a0and c0of the ternary nitride alloy (AlxGa1−xN) are altered by
the Al incorporation, resulting in the change of the spontaneous polarization of AlGaN. The lattice constants and the spontaneous polarization of AlGaN as a function of Al (x)
2
can be expressed as following, [7]
aAlxG a1−xN= −0.077x + 3.189[Å] (2.1)
cAlxG a1−xN= 0.203x + 5.185[Å] (2.2)
Psp(AlxG a1−xN ) = −0.09x − 0.034(1 − x) + 0.021x(1 − x) (2.3)
When a junction is formed between two different semiconductor materials with dis-tinct band gap energies it is called a heterojunction or heterostructure.[8] The AlGaN/GaN heterostructure is formed by a Ga-face 1-5µm GaN buffer layer on a foreign substrate, followed by a 10-30 nm AlGaN barrier layer. The lattice constants of AlGaN is smaller than that of GaN. When an AlGaN layer is grown on top of a GaN buffer, the lattice mis-match is accommodated by some tensile strain in the barrier layer as shown in Figure
2.3. Then, the tensile strain results in the piezoelectric polarization (Ppz) in the AlGaN
layer. In contrast to spontaneous polarization, the piezoelectric polarization is due to externally exerted strain by growth in the crystal structure. This strain causes distortion in the crystal and results in a high strain induced piezoelectric field.[9]
Figure 2.3: Piezoelectric polarization in Ga-face Wurtzite AlGaN due to tensile strain.
The magnitude for piezoelectric polarization in AlGaN along the c-axis can be ex-pressed as,[10] Ppz(AlG aN ) = 2 × aG aN− aAlG aN aAlG aN + (e31− e33 c13 c33 ) (2.4) C13= (5x + 103) × 109[P a] C33= (−32x + 405) × 109[P a] e33= 0.7x + 0.73[C /m2] e31= 0.11x − 0.49[C /m2]
where e13and e33 are the piezoelectric coefficients of the AlxGa1−xN barrier layer, c13
2.2.GAN PROPERTIES
2
17
lattice constant of the GaN layer and relaxed AlGaN layer,respectively. The piezoelectric polarization in the AlGaN barrier layer depends on the Al (x). As the lattice constant of the AlGaN decreases with an increase in Al (x) the piezoelectric polarization increases. For any given, Al (x) as the AlGaN barrier layer exceeds a maximum critical thickness, strain relaxation tends to occur in the crystal structure resulting in reduction of piezo-electric polarization. Therefore, piezopiezo-electric polarization in equation (2.4) can be mod-ified as, [10] Ppz(AlG aN ) = 2[1 − r (x)] aG aN− aAlG aN aAlG aN + (e31− e33 c13 c33 ) (2.5) where, r (x) =aAlG aN ,st r ai ned− aG aN aAlG aN ,r el axed− aG aN
aAlG aN ,st r ai nedand aAlG aN ,r el axedare the lattice constant of AlGaN barrier under stress
and relaxed conditions respectively. The value of (e31− e33cc1333) < 0 is always negative for
the full range of Al (x), therefore under tensile strain (aG aN>aAlG aN) the magnitude of
piezoelectric polarization is always negative and for compressive strain (aG aN<aAlG aN) it
is positive. Since the spontaneous polarization is always negative and points towards the substrate (in Ga-face) for GaN and AlGaN as shown previously, the alignment of sponta-neous and piezoelectric polarization is parallel for tensile stain and anti-parallel for com-pressive strain. Another important feature in III-nitrides is that if the polarity of growth is flipped from Ga-face to N-face or vice versa then both spontaneous and piezoelectric polarizations change directions.[9] The combined effect and constructive combination of spontaneous and piezoelectric polarization in AlGaN/GaN heterostructure with Ga-face growth is illustrated in Figure2.4. The difference in spontaneous and piezoelectric polarizations of two materials causes a high polarization sheet charge density to accu-mulate near the bottom of AlGaN/GaN interface which is illustrated as,
ρPol= PAlG aN–PG aN= PAlG aN ,Spont aneous+ PAlG aN ,P i ezoel ec t r i c–PG aN ,Spont aneous
(2.6) It is noteworthy that GaN buffer layer is fully relaxed and there is no piezoelectric polarization.
2
2.2.2.
2DEG F
ORMINGIf a heterostructure is formed between these two semiconductors then, because of the difference in band gap energies, the conduction (EC) and valance band (EV) cannot be
continuous across the interface as shown in Figure2.5. When the semiconductors are brought together the Fermi level (Ef) aligns and band bending occurs to accommodate
the discontinuity. [8]
A triangular well containing the 2DEG emerges where electrons come from the donor like surface state or bulk GaN material. The maximum sheet carrier concentration formed at the interface of AlGaN/GaN is expressed as, [9]
nx(x) =σx
e − ( ²0²(x)
dde2
)(eφB(x) + EF(x) − ∆Ec(x)) (2.7)
where²0is the electric permittivity,²(x) = 9.5 − 0.5x is the relative permittivity, x is the
Al mole fraction of AlxG a1−xN , ddis the AlGaN layer thickness, eφBis the barrier height
of the gate contact on AlGaN (eφB(x) = 0.84 + 1.3x [eV] ), EFis the Fermi level , ECis the
conduction band (EC(x) = 6.2x +3.4(1−x)−x(1−x))[eV ]) and ∆Cis the conduction band
discontinuity between AlGaN and GaN (∆EC(x) = 0.7[Eg(x) − Eg(0)]), as shown in Table
2-3.
Figure 2.5: Energy Band diagram of AlGaN on Ga-face GaN after heterostructure formation.
The 2DEG sheet charge concentration as mentioned in equation (2.7) increases with the Al content in the barrier layer, because the Al (x) concentration increases both piezo-electric polarization and AlGaN band gap energy resulting in greater conduction band offset∆Ec(x). However, some of the difficulties in making a wider band gap barrier layer
are formation of good quality ohmic contacts and growth strain issues. The 2DEG sheet charge concentration also increases with the AlGaN barrier thickness but only up to a
2.2.GAN PROPERTIES
2
19
maximum critical thickness, typically 40 nm, and then flattens out with further increase in the thickness due to strain relaxation.
Recently, more modified barrier designs have been reported to further improve the AlGaN/GaN HFETs performance such as AlGaN/AlN/GaN structures. The major restric-tions in achieving very high electron mobility in the 2DEG are interface scattering, dis-location scattering and alloy disorder scattering. With the insertion of a very thin ( 1nm) and wide band gap (6.2 eV) AlN material layer sandwiched between the AlGaN and GaN layers as shown in Figure2.6, the conduction band offset,∆Ec, is further increased and
electron alloy disorder scatterings is reduced. The binary AlN spacer layer reduces the penetration of electrons from the GaN channel into the ternary AlGaN barrier layer, thereby significantly improving the electron mobility.
Figure 2.6: Energy Band diagram of AlGaN/AlN/GaN heterostructure and a 2DEG distribution with and with-out a 1 nm AlN interlay.[11]
In summary, the inherent advantage of a heterostructure in HEMTs is the forma-tion of 2DEG, which is formed by the confinement of electrons in a defined triangu-lar quantum well. The electrons have quantized energy levels in a one spatial direction and are only free to move laterally, along the heterostructure interface. This 2DEG has a unique characteristic of extremely high electron mobility ( 2000 cm2V−1s−1) leading
to much reduced on-state resistance (Ron) and improved high frequency performance.
The presence of very high polarization effects in GaN makes it possible to fabricate de-vices without the intentional doping of the upper wide band gap material. This signifi-cantly reduces ionized impurity scattering and Coulomb scattering as the 2D electrons are separated from the supply atoms. Due to these polarization effects the 2DEG sheet charge density in AlGaN/GaN heterostructures is about five times higher than that in doped AlGaAs/GaAs HEMTs. Therefore, AlGaN/GaN heterostructures are widely used
2
in microwave and power device. However, the application of AlGaN/GaN heterostruc-ture based sensor are not fully developed. Next section introduces the basic mechanism and structures of the AlGaN/GaN heterostructure based sensors reported in this thesis, namely gas sensors, UV detectors, and pressure sensors.
2.3.
A
L
G
A
N/G
A
N H
ETEROSTRUCTURE
S
ENSORS
2.3.1.
S
URFACEC
HEMISTRY ANDG
ASS
ENSINGM
ECHANISMSThe most commonly used structures of AlGaN/GaN heterostructure gas sensors are the HEMT and the Schottky barrier diode (SBD) as depicted in Figure2.7. Compared with the tranditional power and microwave HEMT/SBD devices, the main differences are that the gate/anode active area are without passivation and replaced by gas reactive materi-als, such as catalytic metal, metal oxide, polymers, or nano materials. Upon exposure to the gas, the interaction with the functional gate changes the surface state, which leads the change of the sheet carrier concentration of 2DEG channel at the interface of the Al-GaN/GaN heterostructure. This effect is used in gas sensors which are able to detect the surface polarity change by gases or liquids.
Figure 2.7: Schematic image of AlGaN/GaN heterostructure sensors (a)HEMT; (b) Schottky barrier diode.
According to the strength of interaction between the gas molecule and the functional material surface, there are two adsorption phenomena: physisorption and chemisorp-tion. The energy curve along with reaction distance for a gas molecule approaching a material surface is shown in Figure2.8to illustrate the adsorption process. Physisorption is a weak interaction and the bonding is due to van der Waals forces. The physisorption usually happens at relative low temperature and the typical binding energies are 10-100 meV. If the molecules can overcome an activation energy barrier Econv, it will become chemisorption, which involves covalent or ionic bonds. The binding energies are 1-10 eV at a distance of 1-3 Å from the surface. Chemisorption is usually a dissociative pro-cess in which the activation barrier height is related to that energy to dissociate the gas molecules. The activation energy is highly dependent on the material, its surface struc-ture, and molecular orientation. And the adsorption sites of the material surface and the temperature also play a vital role in the chemisorption process. The desorption is the reverse process of adsorption. Therefore, a heating unit for gas sensor is usually needed to speed up the rate of chemisorption and desorption.
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Figure 2.8: The potential energy vs. distance from the material surface. Econv: activation energy barrier height. Ep: physisorption potential well; Ec: chemisorption potential well. [12]
For a bare gate or porous gate, direct adsorption of the gaseous molecules occurs on the top surface of the GaN layer. As these open areas are exposed to an oxygen-containing atmosphere during processing and measurement, the GaN probably is oxi-dized so that the adsorption in the pores takes place on some form of non-stoichiometric Ga-oxynitride. The adsorption leads to a change in the GaN depletion layer. In gen-eral, oxidizing gases lead to an increased depletion layer and a decreased source-drain current, whereas sensor signals appear opposite upon exposure to reducing gases like CO, H2.[13] The modulation of the depletion layer caused by chemisorption of gaseous
species on bare GaN areas results in changes in the electron density of 2DEG and there-fore, in the source-drain current. For different functional materials and different molec-ular species, the basic principle is almost the same, namely the surface state is changed by the gas absorption and then results in the changes of the depletion layer. Therefore, the functional materials play an important role to selectively sensitive behaviour to tar-get gases.
A representative example is the Pt surface on AlGaN/GaN heterostructure for gas de-tection as shown in Figure2.9. Molecular hydrogen adsorbs on the Pt surfaces and the hydrogen species diffuse rapidly through the metal to build up a H-induced dipole layer at the Pt-GaN interface. The dipole moment is oriented out of the GaN leading to a neg-ative voltage drop. The potential drop at the interface is balanced by a modulation of the depletion layer, which leads to a decrease in the barrier height and an increase in the drain current. The change of barrier height based on different gases is illustrated in Figure2.10. The sensing mechanism could be explained at the saturation region of an AlGaN/GaN HEMT as following equations,
IDS=
µCgWg
2Lg
2
Figure 2.9: A representative example: Pt/AlGaN/GaN gas sensor (Ha: absorbed Hydrogen, Hi: interfacial Hy-drogen, Oa: absorbed Oxygen, Oi: interfacial Oxygen). [12]
Figure 2.10: Energy band diagram of a Pt/AlGaN/GaN heterostructure sensor in air (black line) and under hydrogen (red dotted line) or NO2(blue dotted line) gas.
VT= ΦB−∆EC e − ens Cg (2.9) ns=σ e − ( ²0² dde2 )[e(ΦB− VGS) + EF− ∆EC] (2.10)
whereµ is the 2DEG mobility, Wg and Lg are the gate width and length, Cg is the gate
to channel capacitance,ΦB is the barrier height,∆EC is the conduction band
disconti-nuity, e is the elementary charge, nsis the sheet carrier density,σ is the channel density,
²0and² are the electric permittivity aand the relative permittivity of AlGaN layer, dd is
the AlGaN layer thickness, EF is the Fermi level. For metal gate, the barrier height is
related to the work function of the metal (φm) and the semiconductor electron affinity
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difference of the two semiconductors electron affinity,ΦB=χs1-χs2. Also, the sheet
car-rier density changes with the barcar-rier height. The increase of barcar-rier height results in a decrease of the sheet carrier density, which leads to an increase of VT and a decrease of
drain current.
2.3.2.
UV
DETECTIONThe basic principle of semiconductor photo sensor or detector is the internal photo-electric effect. The internal photophoto-electric effect does not produce photoelectrons which can be observed outside the material, but only excites electrons from the valence band to the conduction band in the semiconductor material as shown in Figure2.11. An ex-cited electron and the vacancy the promoted electron leaves behind, are referred to as electron-hole pair. In general, the electron-hole pair will recombine if they are left in the material long enough. This time is called the recombination lifetime,τr. The
elec-trons and holes in impurity energy level also could absorb the photon. Common devices based on the internal photoelectric effect are solar cells and photodetectors (PDs).
Figure 2.11: Schematic drawing of the internal photoelectric effect.
For the intrinsic semiconductor, the absorption condition is according to the follow-ing equation: hv ≥ Eg (2.11) λ ≤hc Eg = 1.24 Eg [µm] (2.12)
where h is the Planck constant, v andλ are the frequency and wavelength of the incom-ing light, respectively; Eg is the bandgap of semiconductor materials. The bandgap of
semiconductor materials should be above 3.1 eV (400 nm) if the photodetectors need to be blind for visible light. For UV detection, the wide-bandgap materials, such as GaN [14–16], AlN[17–19], ZnO [20–22], Ga2O3[23–25], WO3[26–28]and their combinations,
are of great interest. GaN is one of the most promising semiconductors for the UV detec-tion because of its large direct bandgap (3.4 eV at room temperature in wurtzitic struc-ture), high thermal and chemical stability, and resistance in harsh environment condi-tions.
2
Numerous UV photodetectors based on GaN-based materials have been reported in various configurations, such as photoconductor[29–31], phototransistor, metal-semiconductor-metal (MSM) detector [14–16], Schottky diodes[32] and p-n or p-i-n photodiode[33,34]. The most common structures of GaN-based UV photodetector are shown in Figure2.12. An ideal photodetector would exhibit low dark current to minimize the interference noise and high responsivity to maximize the signal. Compared to other photodetector archi-tectures, AlGaN/GaN heterostructure photodetectors are able to provide an extremely large photoconductive gain due to the high electron density and velocity ( 107cm/s) of 2DEG. In other words, the AlGaN/GaN heterostructure have higher responsivity.
Figure 2.12: Typical GaN-based UV photodetector structures: (a) photoconductor; (b) MSM photodetector; (c) Schottky photodiode; (d) PIN photodiode.
The basic mechanism of AlGaN/GaN heterostructure photodetector is illustrated in Figure2.13. Upon exposure to the ultraviolet light (hv > Eg), the electron-hole pairs are
generated in both the AlGaN layer and GaN layer. For the AlGaN layer, the optical gener-ated electrons move into the 2DEG channel due to the built-in polarization field and the holes are swept to the surface. There are several mechanisms to explain the movement of the electron-hole pair generated in the GaN buffer layer [35–37]. Vetury et al[35] pro-posed that the photogenerated holes moved to the surface due to the electric field in the AlGaN layer. Yun et al[38] considered that the movement to the surface occurs because of the thermal energy from the light illumination and neutralization of the surface state. Zaffar et al[37] proposed that the generated holes moved to the GaN/substrate interface due to the barrier height of the valence band ( 0.26 eV) and the direction of the built-in electric field of the GaN layer is towards the substrate. In this thesis, we suggest that the generated holes in the GaN layer are pulled by the built-in electric field to the substrate
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as a virtual back-gate; the electrons move to the 2DEG channel; the photo generated electrons in the AlGaN layer move to the 2DEG channel and the holes are swept to the surface as a virtual top-gate.
Figure 2.13: (a) Schematic diagram of a AlGaN/GaN heterostructure photodetector under illumination show-ing the electron-hole pair generation and movement. (b) Equivalent mechanisms of band gap structure.
The spectral current responsivity of the photodetector is expressed: Rλ=IP
P [A/W ] (2.13)
where Ip is the output current of AlGaN/GaN heterostructure photodetector, P is the
incident power. For the AlGaN/GaN heterostructure photodetector, the drain current is affected by the photocurrent both in the AlGaN layer and the GaN layer. According to the theory of internal photoelectric effect and HEMT device, the change in drain current (∆I = ∆I1+ ∆I2) is expressed by the following equations[37,39]:
∆I1= IPh,AlG aN×τ h τe (2.14) ∆I2= gmnkT q l n[1 + IPh,G aN Is ] (2.15)
where IPh,AlG aNand IPh,G aNare the photocurrent in AlGaN and GaN layer, respectively,
τhandτeare effective holes lifetime on the surface and channel electron lifetime, n, k, T, q
and ISare the ideality factor for the heterostructure, Boltzmann constant, temperature
in K , electronic charge, reverse saturation current, respectively, and gmis the
appropri-ate transconductance for the back-gappropri-ate bias.
2.3.3.
P
RESSURE SENSINGThe piezoelectric and spontaneous polarization properties of AlGaN/GaN-based ma-terials suggests that the AlGaN/GaN heterostructure would be an excellent candidate for a pressure or stress sensors. They can be realized by etching away the substrate to form a moveable MEMS structure and building the AlGaN/GaN sensing unit on it. The schematic diagram of a AlGaN/GaN membrane structure pressure is shown in
Fig-ure2.14. The direction of pressure to membrane depends on the difference pressure
